1. Field of the invention
The present invention relates to an image processing system and an image processing method for aerial photographs, which supports map-making by using photographic images such as aerial photographic images and satellite images and analyzing features shown in these photographic images.
2. Description of the Related Art
In recent years, there has been a tendency for the users of digital map data to increase each year because of the proliferations of car navigation devices mounted on motor vehicles and portable terminals (portable telephones particularly) equipped with a GPS function. For this reason, there has been demanded development of map-making systems, which make, at lower cost, digital map data by which the user can get clearer and more detailed data on their present position and destination, and which is more accurate, more up-to-date, and more increased in the volume of information.
Known methods include the method of making the map data based on aerial photographs. Herein, the map means a topographical-house map, road map, land-use map, land-coverage map, and electronic map for car navigation, and electronic map for a personal computer. The technology of making the above-described maps by using aerial photographs is described under the title of “Technique for Mapping” in “Computer Mapping” (by Sakauchi, Kakumoto, Ohta, and Hayshi, pressed by Shokodo Press, 1992, pp. 45-50).
The above-described conventional method has required the work of tracing the shape of a feature from the lines constituting the outline of the feature, obtained by manually projecting the image of a shadow obtained by a camera having taken a picture in a negative film. However, the advent of digital cameras, the improvement of the number of pixels, and the improvement of sensitivity sensors that detect features from the image of a digital camera have enabled the data used for making a map that is equal to or better than a negative film in the data amount, to be taken-in as digital data.
For this reason, JP-A2004-252213 discloses that features are polygonized and are taken into the database as electronic data by dividing, into line segments, the change boundary of the color information obtained by means of edge extraction from the aerial photographs taken by a high performance digital camera or satellite images.
However, in the aerial photograph or the satellite image taken with natural light, there always exists the shadow of the feature, which is a non-feature and was produced by the natural light. And the shadow is analyzed as the change boundary of the color information obtained by the edge extraction. The shadow makes it impossible to detect the accurate shape of the feature to perform the edge extraction, and the shadow interferes with the polygonization of the feature.
For this reason, JP-A11-283012 discloses that the area consisting the shadows of features is identified from the image by grouping the area of the color information obtained from the edge extraction of the shadow of non-feature, which is always produced in the aerial photograph or the satellite image taken with natural light and using the standard, obtained therefrom, that the area consisting the shadows of features is relatively darker than others.
Further, JP-A6-105224 discloses that the definition of image is increased by means of image editing, which increases the dynamic range of the image-inputting device in order to increase the resolution of the image of the shadow area. However, the method does not eliminate the shadow area from the image, and the method increases the degree of minuteness of the whole image.
As described hereinabove, in recent years, thanks to advances in the technology related to the image photographed by a digital camera and in remote-sensing technology using space satellites, which employs electric wave, light, and infrared rays, it has become possible to make map-data that detected features to high degrees of detail from aerial photographs and satellite images. In particular, an aerial photograph has become proud of its resolution of accuracy up to about 20 cm, and moreover, the improvement of ortho-correction technique has brought about the remarkable enhancement of accuracy in standard practice. Therefore, the automatic generation of the polygon data of the features from the digital image of the aerial photograph and the satellite image has become possible. For this reason, attempt has been made to always obtain the updated map data by timely detecting the change of the feature.
However, when taking aerial photographs or satellite images, which do not enable lighting apparatus to be used, the photographing has no choice but to be done with natural solar light. The improvement of the resolution of today's digital camera and of sensitivity control technique has allowed the mapping worker to visibly confirm and observe the shape of the feature in the image because of the occurrence of the change of color-tone caused by the diffused reflection of solar light on the surface of the earth and the surface of the feature also as to the shadow portion of the aerial photograph and the satellite image. However, it was not possible to automatically recognize, by mechanizing the system, the area that was not directly shined by solar light (referred to as “shadow area” hereinafter) by its being behind the feature, in the image of small difference of color gradation by means of the conventional function of detecting the shape of the feature at the wide gradation area of color, containing an area directly shined by solar light (referred to as “solar irradiation area”) in the image placed on the map-data-making apparatus. For this reason, there is a problem that the shadow area in image prevents the shape of the feature from being accurately recognized from the digital image of the aerial photograph or the satellite image and the shadow area therein interferes with the automatic generation of the polygon data of the shape of the feature.